The present invention pertains to the art of casting and forming composite metal articles, and more particularly to the containment of a molten metal in a metal shell which is engaged by one or more forming dies and formed to a desired shape.
Heretofore, steel and other metals with the desired characteristics of high strength, hardness, and corrosion resistance have been cast in molds of sand or ceramic because of the high temperature melting point of these metals. The molds required for these processes must be made on an individual basis using labor intensive methods. The casting process, itself, is likewise labor intensive and extensive secondary operations to clean and trim the castings are required. The furnaces and attendant equipment must operate at very high temperatures which makes for high equipment cost and high maintenance costs. All of which, when combined with the high cost per pound of heavy casting materials, result in castings of high cost and excessive weight.
The die casting of low melting temperature metals such as aluminum and zinc alloys is a widely used process that addresses some of the problems by utilizing resusable steel dies operated at lower temperatures. While this produces castings of light weight with closer dimensional control and better surface finish, the use of such low melting temperature materials is much restricted by low strength, lack of wear resistance, and poor corrosion resistance.
Attempts have been made to cast steel alloys in metal dies made of very high melting temperature metals in processes similar to that of die casting low melting temperature materials, but the charge of molten steel must be cast at so high a temperature that the surfaces of the dies are severely stressed by thermal shock and the dies breakdown after limited use. Also, the very high melting temperature metals from which the dies are made, are expensive and are difficult to machine into the required die shapes. Accordingly, such processes for casting high temperature metals have not met with wide acceptance.
Efforts have been made to overcome these problems by casting bimetallic products. Some examples are die casting of aluminum around steel inserts; rolling or explosion bonding of aluminum and steel in flat bimetal pieces or sheets; and electroplating or hot dipping a molten metal on a base metal. These methods are each servicable in a limited range of application but are severely limited in one or more areas of performance such as product size and shape, structural strength, and corrosion protection.
Heretofore, methods for casting bimetallic products have only been successful in obtaining a structurally sound metallurgic bond between those metals having nearly the same coefficient of thermal expansion or in those articles in which the metal with the lower coefficient of thermal expansion is positioned interior to the metal with the higher coefficient of thermal expansion. This relative position will cause the outer metal to shrink more upon cooling and in so doing, tighten upon the inner metal to form a secure joint. Composite metal forms in which the metal with the higher coefficient of thermal expansion is the interior metal, result in the interior metal drawing away from the exterior metal thus creating stress and possible voids at the bonding interface of the two metals.
Another limitation of present methods of die casting is that the dies are required to come in direct contact with the molten metal to contain and form it. This requires dies with precise dimensions, smooth surface finish, and full enclosure of the molten metal. Therefore, the dies are expensive.
Yet another problem with present methods is that of removing oxides from the bonding surfaces of the metals to be joined. The highly reactive nature of such metals as aluminum and the high melting temperature of its oxide makes oxide removal and the prevention of its subsequent formation an expensive and difficult operation.